The present invention is directed to polyesters, including unsaturated polymers, particularly useful for preparing low fix temperature toners, for example a toner that fixes below 200.degree. C., and preferably below 160.degree. C., by hot-roll methods, and processes for the preparation thereof. More specifically, the present invention relates to processes for the preparation of polyester resins useful as toner resins and wherein the process comprises the reaction of a linear polyhydroxy functional polyester resin with a dianhydride or diepoxy functional crosslinking component. In embodiments, a linear polyhydroxy functional polyester is crosslinked in the melt with a pyromellitic dianhydride (PMDA) and without the use of peroxides, thereby enabling the desired crosslinked product to form rapidly, and in embodiments within about 3 minutes. The crosslinking reaction of the present invention is rapid, within about 1 to 30 minutes; cost effective, in that it effectively eliminates the vacuum time thus reducing the cost of the final polymer; and further the crosslinking can be accomplished in an extruder in one step and without the use of peroxides. The aforementioned extruder includes the reactive extruder processes as illustrated in U.S. Ser. No. 814,641 (D/91117), and U.S. Pat. No. 5,227,460 (D/91117Q), the disclosures of which are totally incorporated herein by reference. In the aforementioned documents, there are illustrated, for example, reactive extrusion processes for obtaining low melt toner resins comprising linear portions and crosslinked portions, and wherein the crosslinked portions consisting essentially of high density crosslinked microgel particles.
Toner can be fixed to a support medium, such as a sheet of paper or transparency, by different fixing methods. A fixing system which is very advantageous in heat transfer efficiency and is especially suitable for high speed electrophotographic processes is hot roll fixing. In this method, the support medium with a toner image thereon is transported between a heated fuser roll and a pressure roll, with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium forming a fixed image.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is referred to as Cold Offset Temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is referred to as the Hot Offset Temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images resulting, for example, in blurred images. This undesirable phenomenon is referred to as offsetting. Between the COT and HOT of the toner is the Minimum Fix Temperature (MFT), which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, that is, as determined by, for example, a creasing test. The difference between MFT and HOT is referred to as the Fusing Latitude.
The hot roll fixing system described above and a number of toners presently used therein exhibit several problems. First, the binder resins in the toners can require a relatively high temperature for affixing to the support medium. This may result in high power consumption, low fixing speeds, and reduced life of the fuser roll and fuser roll bearings. Image and toner offsetting can also be a problem. Further, toners containing vinyl type binder resins, such as styrene-acrylic resins, may have an additional problem which is known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image comes in contact for a period of time with a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible, such as for example in vinyl binder covers, and the fixed image adheres to the PVC surface.
Many processes are known for the preparation of toner resins like polyesters. For example, the preparation of polyesters by the reaction of an acid and a diol are known, see for example U.S. Pat. No. 3,590,000. Also known are transesterification processes for the preparation of polyesters and reactive extrusion processes for the preparation of crosslinked polyesters. In U.S. Pat. No. 3,681,106, for example, a polyester resin was improved with respect to offset resistance by nonlinearly modifying the polymer backbone by mixing a trivalent or more polyol or polyacid with the monomer to generate branching during polycondensation. However, a high degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished. One method of improving offset resistance is to utilize a crosslinked resin in the binder resin. For example, U.S. Pat. No. 3,941,898 discloses a toner in which a crosslinked vinyl type polymer is used as the binder resin. Similar disclosures for vinyl type resins are located in U.S. Pat. Re. No. 31,072 (a reissue of U.S. Pat. No. 3,938,992), U.S. Pat. Nos. 4,556,624; 4,604,338 and 4,824,750.
Crosslinked polyester binder resins prepared by conventional polycondensation reactions have been prepared for improving offset resistance, such as for example in U.S. Pat. No. 3,681,106. As with crosslinked vinyl resins, increased crosslinking as obtained in such conventional polycondensation reactions may cause the minimum fix temperature to increase. When crosslinking is carried out during polycondensation using tri or polyfunctional monomers as crosslinking agents with the polycondensation monomers, the net effect is that apart from making highly crosslinked high molecular weight gel particles, which are not soluble in substantially any solvent, the molecular weight distribution of the soluble part widens due to the formation of sol or crosslinked polymer with a very low degree of crosslinking, which is soluble in some solvents. These intermediate high molecular weight species may result in an increase in the melt viscosity of the resin at low and high temperature, which can cause the minimum fix temperature to increase. Furthermore, gel particles formed in the polycondensation reaction, which is carried out using conventional polycondensation in a reactor with low shear mixing, can grow rapidly with increase in degree of crosslinking. As in the situation with crosslinked vinyl polymers using conventional polymerization reactions, these large gel particles may be more difficult to disperse pigment in, resulting in unpigmented toner particles after pulverization, and thus hindering developability.
Electrophotographic toners are generally prepared by mixing or dispersing a colorant and possibly a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. Known conventional thermoplastic binder resins include polystyrenes, styreneacrylic resins, styrene-methacrylic resins, certain polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. Carbon black is often used as a colorant and alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfates, and the like are employed as charge enhancing additives.
U.S. Pat. No. 4,533,614 discloses a nonlinearly modified, low-melting polyester containing: 1) an alkyl-substituted dicarboxylic acid and/or an alkyl-substituted diol; 2) a trivalent or more polycarboxylic acid and/or a trivalent or more polyol; 3) a dicarboxylic acid; and 4) an etherated diphenol. The main acid component of the polyester requires 50 mole percent, preferably 60 mole percent, or more of an aromatic dicarboxylic acid, its analogous anhydride, or other dicarboxylic acids to impart sufficient electrophotographic charge characteristics to a toner made from the resin.
U.S. Pat. No. 5,015,724 discloses a modified polyester produced by adding a monoanhydride monomer of 1,2,4-benzene tricarboxylic acid anhydride to a low molecular weight polyester.
U.S. Pat. No. 3,846,375 discloses polymers containing an oxetene (oxacyclobutane) ring attached to a carbon of the aliphatic chain and crosslinked by reactions to those used to crosslink epoxy resins, rendering totally insoluble coating films.
Illustrated in U.S. Pat. No. 5,436,103, the disclosure of which is totally incorporated herein by reference, is a toner with a modified unsaturated linear polymer, which polymer has a glass transition temperature ranging from about 54.degree. C. to about 64.degree. C., and comprising a) a first residue of a first monomer, which first monomer is selected from the group consisting of diacids, anhydrides, diacid esters and mixtures thereof, and the first residue being present in a concentration not less than about 7.5 mole percent, based on the total mole ratio of the polymer; b) a second residue of a second monomer, which second monomer is selected from the group consisting of diols and glycols; and c) an acid residue of an acid monomer, which acid monomer being a substituted aromatic dicarboxylic acid, different from the first residue, and wherein the acid residue is present in a concentration from about 2.5 mole percent to about 12.5 mole percent based on the total mole ratio of the monomer composition.
There is a need for toners which melt at lower temperatures than a number of commercially used toners. Temperatures of approximately 160.degree. to 200.degree. C. are often selected to fix toner to a support medium such as a sheet of paper or transparency to create a developed image. This high temperature may reduce or minimize the life of certain fuser rolls, such as those made of silicone rubbers or fluoroelastomers, such as VITON.RTM., may limit fixing speeds, may necessitate larger amounts of power to be consumed during operation of a copier or printer, such as a xerographic copier which employs a method of fixing such as, for example, hot roll fixing. These and other disadvantages are avoided or minimized with the toners of the present invention.